We discovered LZAP as a novel binding partner of the alternate reading frame protein ARF (p14ARF in humans, p19ARF in mice). Although ARF has p53-independent activities, its major tumor suppressor activity has been attributed to inhibition of MDM2 with resultant activation of p53. We found that binding of LZAP to ARF is increased following oncogenic stimulation and that LZAP increases p53 transcriptional activity both dependent and independent of ARF. Our lab also described that LZAP binds RelA, decreases RelA phosphorylation, inhibits NF-?B transcription, and increases RelA association with histone deacetylases. LZAP has no conserved enzymatic domains and no known enzymatic activity, so mechanisms of LZAP regulation of RelA phosphorylation and activity are currently unknown. We also found that LZAP protein is lost in ~30% of human head and neck squamous cell carcinomas and that loss of LZAP increases anchorage independent growth, invasion, and in vivo xenograft tumor growth. Another group has shown that LZAP sensitizes cells to genotoxic therapeutic agents mediated, at least in part, by LZAP abrogation of the G2/M checkpoint through binding and inhibition of Chk1 and Chk2. Human tumor and xenograft mouse tumor data, as well as, LZAP activities as an activator of p53 and suppressor of RelA suggest that LZAP may function as a tumor suppressor; however, validation of LZAP tumor suppressor status is needed. We now have data that targeting of LZAP in mice results in lung cancer formation. To better understand LZAP biological activities and to gain mechanistic insight, additional LZAP binding partners have been sought. We have recently found and confirmed that LZAP binds the stress activated protein kinase, p38 MAPK (hereafter p38), and the wild-type p53 induced phosphatase, Wip1. Both tumor suppressor and oncogenic activities have been ascribed to p38 depending on cellular context. Initial exploration revealed that LZAP inhibited p38 phosphorylation and activity (see preliminary data). Wip1 (PPM1D, PP2C4) is a phosphatase whose levels are increased after activation of p53. Wip1 has oncogenic activity as a feedback inhibitor of p53; however, discovery that Wip1 also targets and inhibits RelA suggest that its cellular activity may be more complex. A unifying mechanism of LZAP activity is not defined. Data from our lab for both RelA and p38 and from the Li lab for Chk1/2 suggest that a common effect of LZAP is to decrease phosphorylation of bound proteins. We hypothesize that targeting LZAP in mice will establish it as a tumor suppressor. Mechanistically, we hypothesize that LZAP will exert tumor suppressor activity by regulation of binding partners and p53, and that this regulation will be mediated, at least partially, through the Wip1 phosphatase. We propose to test these hypotheses in this grant.